Backplates for display panels, display panels embodying such backplates, and methods for producing the backplates.
Flat panel, information displays may be of an emissive or non-emissive nature. Emissive displays, such as electroluminescent and plasma displays, characteristically respond to stimulation from an external source. This stimulation makes or modifies the light that they emit to present an image for viewing. Non-emissive displays, such as liquid crystal displays, modulate light from an external source.
Both types of displays essentially consist of front and backplates. Active, structured layers of material between the plates generate, or modulate, light.
Transmissive, non-emissive displays require high optical transparency in both plates. This permits passage of light from an external source. Emissive displays, as well as reflective, non-emissive displays, also require a front plate of high optical transparency to permit passage of light for viewing. Glass sheets are typically used for front plates in each type of display panel.
The backplate for an emissive, or a reflecting, non-emissive display, however, does not need to transmit light. Therefore, it need not be transparent. Indeed, to enhance the light emitted through the front plate for viewing, it can be advantageous to have a reflective backplate.
In any display, the front and backplates are sealed together, often at a relatively high temperature. Further, in many applications, material patterns on the two plates must remain in registration over a range of temperatures. Therefore, it has become common practice to form both plates of the same material, for example, the same glass. This ensures a good match of thermal expansions when a seal is made.
The present invention arose in connection with electroluminescent (EL) displays. Accordingly, particular attention is given to such displays, and to the solution of problems in their production. However, the broader applicability of the invention to other displays will become apparent.
An electroluminescent display consists of an electroluminescent phosphor layer sandwiched between two conducting electrodes. At high voltages, a form of breakdown occurs which causes currents to pass through the phosphor. As a consequence, the phosphor emits light.
Voltages tend to be quite high, that is, greater than 100 volts. Since the phosphor layers are quite thin, the electric fields are very high. To limit current, the displays are typically operated on alternating currents by inserting a dielectric, insulating layer. Current passes on each half-cycle until the capacitance of the device is charged.
The capacitance is proportional to the dielectric constant of the material divided by the thickness of the layer. Therefore, with a material having a high dielectric constant, the thickness of the layer can be greater. This is beneficial since the thicker layer is less prone to manufacturing defects, such as pinholes.
Present EL display panels have row and column electrodes arranged orthogonally with respect to each other. These electrodes are connected to drivers through contact at the periphery of an insulating substrate. Each pixel, then, is defined by a row and column intersection.
Traditionally, EL displays have been fabricated on ceramic or glass substrates. Glass substrates provide the required electrically insulating characteristics, but the transparency provided by glass is unnecessary in the backplate of an EL display panel. Also, glasses are generally not sufficiently refractory to withstand the temperatures involved in material processing.
Consequently, the requirements of an EL display panel are somewhat different from those of a non-emissive display panel. The active materials are formed on the backplate, for example by silk-screening, and are not environmentally sensitive. The front plate essentially acts as a shield against damage to the active material, and no accurate registration needs to be maintained between the plates. With the need for a thermal expansion match relaxed, the front and backplates may be bonded together with a simple, compliant, polymer material.
The manufacture of an inorganic, EL display panel typically involves one of two processes, depending on the thickness of the active material layer. In one process, a thin film is vacuum deposited on the plate surface, and this is followed by an annealing step. The other process involves silk-screening a thick film on the plate and firing to produce an adherent layer. Either process, requires that the back plate, upon which the material is applied, withstand a high temperature, albeit for a short time. Typically the cycle is about 850xc2x0 C. for about fifteen minutes.
Sheets of ordinary glass are not sufficiently refractory to withstand such processing temperatures. As used herein, xe2x80x9crefractoryxe2x80x9d means that a material is capable of withstanding a temperature on the order of 850xc2x0 C. without undergoing destructive, chemical or physical change, or distortion.
The problem just noted with glass has led to use of high temperature ceramics, since transparency is not required. For example, tape cast, alumina sheets have been employed as backplates. Also, vitreous silica has been proposed. Except for the latter, glasses generally lack the required refractoriness.
It is difficult and expensive to manufacture either sintered alumina or vitreous silica sheets. When the sheet size has a diagonal measurement greater than about 20 cm. (8 inches), the process becomes prohibitively expensive. Also, such large alumina sheets tend to be insufficiently flat for silk-screening, or other patterning processes. Vitreous silica has a very low CTE. This makes it difficult to fire a thick film pattern on the sheet without cracking. A CTE greater than 40xc3x9710xe2x88x927/xc2x0 C., and preferably in the range of 40-100xc3x9710xe2x88x927/xc2x0 C., is considered necessary.
The desire for larger EL display panels makes it imperative that an alternative, substrate material be provided. It is a basic purpose of the present invention to meet this need. Another purpose is to provide a novel backplate for an emissive display panel. A further purpose is to provide a backplate for an EL display panel that is readily produced in relatively large sizes. A still further purpose is to provide a backplate for an EL display panel that is mechanically rugged, and that is sufficiently refractory to withstand a processing temperature of at least 850xc2x0 C.
The invention resides, in part, in a backplate for a display panel comprising a thin layer of a glass-ceramic that receives the active display material on its surface, the glass-ceramic being sufficiently refractory to withstand a processing temperature of at least 850xc2x0 C., that has a coefficient of thermal expansion greater than 40, but not over about 100xc3x9710xe2x88x927/xc2x0 C., and that has a crystal phase selected from spinel, enstatite, alpha-quartz, beta-quartz, sapphirine, forsterite, cristobalite, sanbornite, wollastonite, diopside, mullite and mixtures of these crystal phases.
The invention also resides in a display device comprising a display panel having a backplate as just described.
The invention further resides in a method of producing a backplate for a display panel which comprises melting the precursor glass for a glass-ceramic that has a crystal phase selected from spinel, enstatite, alpha-quartz, beta-quartz, sapphirine, forsterite, cristobalite, sanbornite, wollastonite, diopside, mullite and mixtures of these crystal phases, forming a sheet of said glass, heat treating the glass sheet to produce uniform crystallization throughout the sheet.